Magnesium aluminate anion exchangers

ABSTRACT

Preparations and uses are shown for novel crystalline aluminates which conform generally to the empirical formula 
     
         MgA.sub.a.sup.v Z.sub.b.sup.v.nAl(OH).sub.3.mH.sub.2 O 
    
     wherein 
     A and Z represent negative-valence ions or radicals selected from the group comprising hydroxyl, halide, inorganic acid, and organic acid, 
     n is a value of from about 1 to about 2, 
     v is a negative valence of 1, 2, or 3, 
     a and b each have values of from zero to 2, with (va)+(vb) equal to 2, and with 
     m being a value of zero or more.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 183,907 filed Sept. 4,1980, now U.S. Pat. No. 4,326,961, which is a continuation-in-part ofour copending Ser. No. 071,920 filed Aug. 31, 1980, which is adivisional of Ser. No. 939,544 filed Sept. 5, 1978, now U.S. Pat. No.4,183,900, which is, itself, a divisional of Ser. No. 812,542 filed July5, 1977, now U.S. Pat. No. 4,116,857.

BACKGROUND OF THE INVENTION

In our U.S. Pat. Nos. 4,116,857 and 4,183,900 it is shown thatmicrocrystalline MgX₂.2Al(OH)₃, where X is halide, prepared as adispersion in situ within a particulate macroporous anion exchange resinis useful in selectively removing Mg⁺⁺ values from brines. It isunderstood, of course, that the crystals may contain waters ofhydration. Additional embodiments, preparations, and uses of thecrystalline MgX₂.2Al(OH)₃ are the principal subjects of thisapplication.

Aside from the many naturally-occurring aluminates, there aresynthesized metal aluminates such as found, e.g., in U.S. Pat. Nos.2,395,931, 2,413,184, 3,300,577 and 3,567,472.

SUMMARY OF THE INVENTION

Crystalline MgX₂.2Al(OH)₃ is prepared by reacting MgX₂ (where X=halide,especially chloride) with Al(OH)₃ at elevated temperature. The so-formedcrystalline MgX₂.2Al(OH)₃ is contacted with alkali metal hydroxide (esp.NaOH) to form novel crystalline Mg(OH)₂.nAl(OH)₃.mH₂ O. TheMg(OH)₂.nAl(OH)₃ is reacted with negative radicals or anions to formnovel crystalline MgAZ.nAl(OH)₃.mH₂ O compounds, where A and Z representnegative radicals or anions selected from the group comprising ofhydroxyl, halide, inorganic acids, and organic acid as disclosed ingreater detail below.

DETAILED DESCRIPTION OF THE INVENTION

A carrier or medium is provided with alumina hydrate, Al(OH)₃, dispersedtherein. The Al(OH)₃ is then reacted with brine containing MgX₂ atelevated temperature to form crystalline MgX₂.2Al(OH)₃, where X is ahalide, preferably chlorine. When the microcrystalline MgX₂.2Al(OH)₃ isformed and dispersed in situ in a particulate, reticular anion exchangeresin, the composite is particularly useful in selectively removing Mg⁺⁺values from brines (as per U.S. Pat. Nos. 4,116,857 and 4,183,900).

The crystalline MgX₂.2Al(OH)₃ when prepared as a free precipitate, whensupported on a substantially inert substrate, or when supported by (ordispersed in) a cation exchange resin, is useful as an exchanger ofnegative radicals or anions, such as hydroxyl ions, halide anions,inorganic acid anions, or organic acid anions, thereby forming novelcompounds of the general formula MgAZ.nAl(OH)₃.mH₂ O where A and Z aremonovalent anions of the group consisting of hydroxyl ions, halide ions,inorganic acid anions, and organic acid anions.

Throughout this disclosure X is employed to denote halide ions in thecrystalline MgX₂.2Al(OH)₃ which serves as the precursor to thecrystalline Mg(OH)₂.nAl(OH)₃. The symbols A and Z are employed to denoteanions which then replace part or all of the hydroxyl groups attached toMg in the crystalline Mg(OH)₂.nAl(OH)₃. Any of these products maycontain waters of hydration.

The present invention provides new and useful crystalline magnesiumaluminates conforming generally to the empirical formula

    MgA.sup.v.sub.a Z.sup.v.sub.b.nAl(OH).sub.3.mH.sub.2 O

where

A and Z represent negative-valence ions or radicals selected from thegroup comprising hydroxyl, halide, inorganic acid, and organic acid,

n is a value of from about 1 to about 2,

v is a negative valence of 1, 2, or 3,

a and b are each values of from zero to 2, with (va)+(vb) equal to 2,and with

m being a value of zero or more.

Throughout this disclosure, the waters of hydration are not always shownin the empirical formulas, but unless the compounds have beendehydrated, some waters of hydration are most likely present.

Of special interest is the reaction of the crystalline MgX₂.2Al(OH)₃,where X is halide (especially chlorine), with NaOH in sufficientquantity at elevated temperature to form novel crystallineMg(OH)₂.nAl(OH)₃.mH₂ O which is found to undergo exchange with anions toform novel crystalline MgAZ.nAl(OH)₃.mH₂ O compounds.

The so-formed crystalline MgAZ.nAl(OH)₃ compounds are useful as sourcesof the radicals or anions (A and/or Z) by chemically or thermallyreleasing such anions in a desired media. Also, the novelMg(OH)₂.nAl(OH)₃ and MgAZ.nAl(OH)₃ compounds are useful in forming MgAl₂O₄ spinel structures at very high temperatures.

As disclosed in our U.S. Pat. Nos. 4,116,857 and 4,183,900, referred tosupra and incorporated herein by reference, the crystallineMgX₂.2Al(OH)₃ (X=halide) may be prepared within the reticules of aparticulate anion exchange resin, e.g., by the following steps:

1. combine the resin with aqueous AlCl₃ to obtain penetration of theAlCl₃ into the reticules of the resin;

2. convert the AlCl₃ to Al(OH)₃ by reaction with, e.g., NH₄ OH, washingout extraneous material; and

3. add MgX₂ (e.g. MgCl₂) and heat at elevated temperature for a timesufficient to form crystalline MgX₂.2Al(OH)₃.

In the context of this present disclosure, the crystalline MgX₂.2Al(OH)₃may also be obtained by alkaline precipitation of AlCl₃ in an aqueouscarrier or medium. The aqueous medium may also contain other liquid ordissolved materials which may be tolerated so long as they do notprevent the precipitation of the Al(OH)₃ or do not contaminate theAl(OH)₃ to such an extent that subsequent formation of reasonably pureMgX₂.2Al(OH)₃ is substantially deterred.

The aqueous media may also contain non-dissolved substances, such as ionexchange resins (cationic or anionic), inorganic particles, or organicpolymers which may be reticular or substantially non-reticular, andwhich may be found to be beneficial so long as they are substantiallyinert with respect to the reactions involved in the formation of Al(OH)₃or of the subsequent formation of crystalline MgX₂.2Al(OH)₃. Thesenon-dissolved substances beneficially serve as substrates onto which(and into which) the MgX₂.2Al(OH)₃ crystals may form. Thesenon-dissolved substances which shall be referred to herein as"substrates", may be porous or non-porous ion exchange resions (anionicor cationic), may be organic (including polymers), or may be inorganicsuch as stable metal oxides. The amount of crystalline MgX₂.2Al(OH)₃which may be deposited on a given weight of substrate is largelydependent on the surface area of the substrate, therefore it isgenerally best if the substrate is in finely divided form. It isevident, of course, that the substrate selected should be one which willwithstand the heating steps involved in the formation of the MgX₂.2Al(OH)₃ and withstand any subsequent desired reactions withoutinterfering to any substantial extent with the desired reactions. Thesubstrate, then, should be one which is substantially inert andsubstantially stable in the operation of the present invention. In thoseinstances where the MgX₂.2Al(OH)₃ is converted to MgAZ.nAl(OH)₃ and/orMg(OH)₂.nAl(OH)₃ which, in turn, is to be converted at high temperatureto spinels, then selection of substrate, if any, requires considerationof the effect of high temperature on such substrate.

The following examples are to illustrate the practice of the presentinvention, but the invention is not limited to the particularembodiments illustrated.

EXAMPLE 1

An amorphous form of Al(OH)₃, 0.15 mol quantity, is precipitated byadding NH₄ OH to an aqueous solution of AlCl₃. To the so-formed Al(OH)₃slurry is added 0.6 mole of MgCl₂. The mixture is heated to 70° C.,adding NH₄ OH as needed to hold the pH up to 6.7 or more, then filteredand washed with NH₄ OH, then H₂ O. The so-formed MgCl₂.2Al(OH)₃ isredispersed with 5% aqueous solution of MgCl₂ (to assure more completeconversion of the Al(OH)₃) and heated to 95° C. overnight. Then filter,wash ppt. on filter with equimolar mixture of NH₄ Cl/NH₄ OH, then withNH₄ OH. Crystallinity of MgCl₂.2Al(OH)₃ is confirmed by X-raydiffraction pattern.

To the above product is added NaOH (1 N solution) to pH 11, and heatedto 80° C. with stirring to disperse the solids, This is filtered, thesolids combined with more NaOH (1 N solution) to pH 11, heated to 80°C., and refiltered. The NaOH reaction step is repeated 3 times as aprecaution to assure substantially complete conversion toMg(OH)₂.2Al(OH)₃. Then the filtered material is redispersed in H₂ O,heated, and filtered, then re-diluted wit H₂ O and heated at 95° C.overnight to assure substantially complete crystallization. Upon passingthe slurry through a mixed bed of Dowex 50-H⁺ and Dowex 1-OH⁻ ionexchange resins ("Dowex" is a Tradename of The Dow Chemical Company) ona coarse glass fritted filter, the slurry particles are found to be fineenough to pass through. The pH at this point is about 10.4. The slurryis evaporated to 400 ml. volume and by titration of 10 ml. with N/1 HClis found to contain 1.22 mmol Al per 10 ml. of slurry.

Portions of the so-formed stable dispersion of crystallineMg(OH)₂.2Al(OH)₂ (confirmed by x-ray diffraction pattern) are found tobe neutralized by the following acid ions: Cl⁻, ClO₄ ⁻, H₂ PO₄ ⁻, HCrO₄⁻, SO₄ ⁻⁻, NO₃ ⁻, HPO₄ ⁻⁻, and the anion of 1-ascorbic acid, aceticacid, citric acid, and oxalic acid.

The above monovalent acid anions are represented by Z in the empiricalformula Mg(OH)Z.nAl(OH)₃, the divalent anions are represented by Z inthe empirical formula MgZ.nAl(OH)₃ and the trivalent anions arerepresented by Z in the empirical formula MgZ_(2/3).nAl(OH)₃. When allthe OH ions are replaced by monovalent anions the empirical formula isMgZ₂.nAl(OH)₃. In MgAZ.nAl(OH)₃, A and Z may be the same monovalent ionsor different monovalent ions.

EXAMPLE 2

AlCl₃ (0.28 moles) in 900 ml. H₂ O is precipitated as Al(OH)₃ byaddition of NH₄ OH. MgCl₂ (1.0 mole) in water is added, with enough NaOHadded to bring pH to 6.7 (neutral or barely basic). Heating theresulting ppt. at 70° C. causes crystallization. The MgCl₂.2Al(OH)₃crystals are filtered out, washed with dil. NH₄ OH, then with H₂ O.Enough MgCl₂ (5% in H₂ O) is added to moisten the ppt. to give excessMgCl₂. Overnight heating at 95° C. and NaOH neutralization assuressubstantially complete crystalization (confirmed by X-ray) ofMg(OH)₂.2Al(OH)₃. Demineralize with Dowex-50 cation exchange resin totake out excess Cl⁻. At this point the fine particles ofMg(OH)₂.2Al(OH)₃ are easily dispersed in water to form a dispersion.

The Mg(OH)₂.2Al(OH)₃ is converted to Mg(OH)(HCO₃).2Al(OH)₃ by reactionwith CO₂. The x-ray pattern is essentially the same.

EXAMPLE 3

Mix 625 gms. of aqueous slurry containing 2.15 mol. amorphous Al(OH)₃with 172 gms. of aqueous slurry containing 1.075 mol of Mg(OH)₂(brucite) and add 74 mmol of NaOH (as 50% aqueous solution). Warming to70° C. causes thickening; add 200 ml. of H₂ O to thin and heat to 95° C.in oven for 16 hours. Resulting thick paste is stirred into water. Asample of the slurry is filtered, washed, and analyzed by X-ray and itis found that Mg(OH)₂.2Al(OH)₃ is the major crystalline component with atrace of bayerite (or gibbsite) and some amorphous Al(OH)₃ and Mg(OH)₂.The remaining slurry is put back into 95° C. for 24 hours and X-rayshows more crystalline Mg(OH)₂.2Al(OH)₃ with a small amount of boehmite.

A small disc (about 22 diameter and 7.5 mm thick) when pressed and firedat high temperature (>1200° C.) is found to have formed spinel, MgAl₂O₄, and to have a density of about 94% of theoretical.

There are, of course, many organic acids and inorganic acids whichcontain anions which are operable in the present invention. The largerthe anion, the greater is the possibility of expanding the crystallattice structure to the limit, while still maintaining thecrystallinity. For example, with 1-ascorbic acid (vitamin C) which is a6-carbon molecule, the Mg(OH)Z.2Al(OH)₃ crystal is considerably expandedby the large anion (Z) which has replaced (exchanged with) one of theOH⁻ ions. Other organic acids having from 1 to about 8 carbon atoms arewithin the purview of this invention, including monobasic, dibasic, andtribasic carboxylic acids.

The various novel compounds of the present invention may be used informing spinels, MgAl₂ O₄, at high temperature and are also useful inexchanging with other anions in various solutions. For instance,ascorbic acid groups may be released from the present crystallinecompounds into solutions which contain other anions, such as Cl⁻, whichcan replace the ascorbic acid in the crystal.

We claim:
 1. A method for removing negative-valent ions or radicals fromaqueous medium, said method comprisingcontacting said aqueous mediumwith crystalline Mg(OH)₂.nAl(OH)₃.mH₂ O, where n is a value of fromabout 1 to about 2 and m is a value of zero or more, and where thecrystalline Mg(OH)₂.nAl(OH)₃.mH₂ O is supported by a substrate, therebyexchanging OH ions in the said crystalline material with the saidnegative-valence ions or radicals.
 2. The method of claim 1 wherein thecrystalline Mg(OH)₂.nAl(OH)₃.mH₂ O is supported by a porous or reticularsubstrate.
 3. The method of claim 1 wherein the crystallineMg(OH)₂.nAl(OH)₃.mH₂ O is supported by an inorganic substrate.
 4. Themethod of claim 1 wherein the crystalline Mg(OH)₂.nAl(OH)₃.mH₂ O issupported by an organic or polymeric substrate.
 5. The method of claim 1wherein the crystalline Mg(OH)₂.nAl(OH)₃.mH₂ O is supported by an ionexchange resin.
 6. The method of claim 1 wherein the crystallineMg(OH)₂.nAl(OH)₃.mH₂ O is supported by a substrate which is in a liquidmedium.
 7. The method of claim 1 wherein the crystallineMg(OH)₂.nAl(OH)₃.mH₂ O is supported by a substrate which is in anaqueous medium.